This invention relates to radio broadcasting, and more particularly, to modulation formats for increasing data capacity in Digital Audio Broadcasting (DAB) systems utilizing a orthogonal frequency division multiplexing.
Digital Audio Broadcasting is a medium for providing digital-quality audio, superior to existing analog broadcasting formats. Both AM and FM DAB signals can be transmitted in a hybrid format where the digitally modulated signal coexists with the currently broadcast analog AM or FM signal, or in an all-digital format without an analog signal. In-band-on-channel (IBOC) DAB systems require no new spectral allocations because each DAB signal is simultaneously transmitted within the same spectral mask of an existing AM or FM channel allocation. IBOC promotes economy of spectrum while enabling broadcasters to supply digital quality audio to their present base of listeners. Several IBOC DAB approaches have been suggested. One such approach, set forth in U.S. Pat. No. 5,588,022, presents a method for simultaneously broadcasting analog and digital signals in a standard AM broadcasting channel. Using this approach, an amplitude-modulated radio frequency signal having a first frequency spectrum is broadcast. The amplitude-modulated radio frequency signal includes a first carrier modulated by an analog program signal. Simultaneously, a plurality of digitally-modulated carrier signals are broadcast within a bandwidth which encompasses the first frequency spectrum. Each digitally-modulated carrier signal is modulated by a portion of a digital program signal. A first group of the digitally-modulated carrier signals lies within the first frequency spectrum and is modulated in quadrature with the first carrier signal. Second and third groups of the digitally-modulated carrier signals lie outside of the first frequency spectrum and are modulated both in-phase and in-quadrature with the first carrier signal. Multiple carriers are employed by means of orthogonal frequency division multiplexing (OFDM) to bear the communicated information.
FM IBOC broadcasting systems using have been the subject of several United States patents including U.S. Pat. Nos. 5,465,396; 5,315,583; 5,278,844 and 5,278,826. In addition, a commonly assigned pending patent application for a "Method and System for Simultaneously Broadcasting and Receiving Digital and Analog Signals, by D. Kumar and B. Hunsinger, Ser. No. 08/294,140, filed July 1994 discloses an FM IBOC DAB system. These FM IBOC DAB systems utilize a plurality of OFDM sub-carriers to transmit digitally encoded audio and data signals.
One way in which data can be encoded for transmission is amplitude-shift-keying (ASK) in which the amplitude of a carrier is varied in accordance with the level of the data. Another way to encode data is to designate an array of data points in a complex plane and then assign each of these points to represent a particular pattern of digital information, or symbol. These data points can be described by either real and imaginary, or phase and amplitude, numbers. Each such array of data points is called a constellation. Using this encoding scheme, a received signal can be related to specific symbols in the constellation, and identified with the original information transmitted. Phase-shift-keying (PSK) is such an encoding scheme, in which the modulating function shifts the instantaneous phase of the carrier among predetermined discrete symbols. The information, therefore, can reside in the phase of the carrier. From the perspective of noise immunity, coherent PSK can be superior to other schemes. Modulated signals can carry information in the symbol's amplitude as well as its phase, further increasing the information carried by any particular symbol. However, in practice, the number of symbols able to be clustered into a particular constellation is limited by factors such as symbol rate and channel distortion. Factors such as thermal noise, intersymbol interference, random and non-linear channel effects, and the like, (collectively, "distortion") can corrupt both the phase and the amplitude of the transmitted signal waveform, thereby tending to reduce the separation between neighboring symbols.
In IBOC DAB systems which operate channels that span a limited range of frequencies, it is desirable to utilize modulation formats that can deliver sufficient data throughput to provide acceptable audio quality and/or data delivery. This invention seeks to increase the data capacity of such DAB systems.